Earth-boring bits may have fixed or rotatable cutting elements. Earth-boring bits with fixed cutting elements typically include a bit body machined from steel or fabricated by infiltrating a bed of hard particles, such as cast carbide (WC+W2C), macrocystalline or standard tungsten carbide (WC), and/or sintered cemented carbide with a copper-base alloy binder. Conventional fixed cutting element earth-boring bits comprise a one-piece bit body with several cutting inserts in insert pockets located on the bit body in a manner designed to optimize cutting. It is important to maintain the inserts in precise locations to optimize drilling efficiency, avoid vibrations, and minimize stresses in the bit body in order to maximize the life of the earth-boring bit. The cutting inserts are often based on highly wear resistant materials such as diamond. For example, cutting inserts may consist of a layer of synthetic diamond placed on a cemented carbide substrate, and such inserts are often referred to as polycrystalline diamond compacts (PDC). The bit body may be secured to a steel shank that typically includes a threaded pin connection by which the bit is secured to a drive shaft of a downhole motor or a drill collar at the distal end of a drill string. In addition, drilling fluid or mud may be pumped down the hollow drill string and out nozzles formed in the bit body. The drilling fluid or mud cools and lubricates the bit as it rotates and also carries material cut by the bit to the surface.
Conventional earth-boring bit bodies have typically been made in one of the following ways, for example, machined from a steel blank or fabricated by infiltrating a bed of hard carbide particles placed within a mold with a copper based binder alloy. Steel-bodied bits are typically machined from round stock to a desired shape, with topographical and internal features. After machining the bit body, the surface may be hard-faced to apply wear-resistant materials to the face of the bit body and other critical areas of the surface of the bit body.
In the conventional method for manufacturing a bit body from hard particles and a binder, a mold is milled or machined to define the exterior surface features of the bit body. Additional hand milling or clay work may also be required to create or refine topographical features of the bit body.
Once the mold is complete, a preformed bit blank of steel may be disposed within the mold cavity to internally reinforce the bit body matrix upon fabrication. Other transition or refractory metal based inserts, such as those defining internal fluid courses, pockets for cutting elements, ridges, lands, nozzle displacements, junk slots, or other internal or topographical features of the bit body, may also be inserted into the cavity of the mold. Any inserts used must be placed at precise locations to ensure proper positioning of cutting elements, nozzles, junk slots, etc., in the final bit.
The desired hard particles may then be placed within the mold and packed to the desired density. The hard particles are then infiltrated with a molten binder, which freezes to form a solid bit body including a discontinuous phase of hard particles within a continuous phase of binder.
The bit body may then be assembled with other earth-boring bit components. For example, a threaded shank may be welded or otherwise secured to the bit body, and cutting elements or inserts (typically diamond or a synthetic polycrystalline diamond compact (“PDC”)) are secured within the cutting insert pockets, such as by brazing, adhesive bonding, or mechanical affixation. Alternatively, the cutting inserts may be bonded to the face of the bit body during furnacing and infiltration if thermally stable PDC's (“TSP”) are employed.
The bit body and other elements of earth-boring bits are subjected to many forms of wear as they operate in the harsh down hole environment. Among the most common form of wear is abrasive wear caused by contact with abrasive rock formations. In addition, the drilling mud, laden with rock cuttings, causes the bit to erode or wear.
The service life of an earth-boring bit is a function not only of the wear properties of the PDCs or cemented carbide inserts, but also of the wear properties of the bit body (in the case of fixed cutter bits) or conical holders (in the case of roller cone bits). One way to increase earth-boring bit service life is to employ bit bodies made of materials with improved combinations of strength, toughness, and abrasion/erosion resistance.
Recently, it has been discovered that fixed-cutter bit bodies may be fabricated from cemented carbides employing standard powder metallurgy practices (powder consolidation, followed by shaping or machining the green or presintered powder compact, and high temperature sintering). Such solid, one-piece, cemented carbide based bit bodies are described in U.S. Patent Publication No. 2005/0247491.
In general, cemented carbide based bit bodies provide substantial advantages over the bit bodies of the prior art (machined from steel or infiltrated carbides) since cemented carbides offer vastly superior combinations of strength, toughness, as well as abrasion and erosion resistance compared to steels or infiltrated carbides with copper based binders. FIG. 1 shows a typical solid, one-piece, cemented carbide bit body 10 that can be employed to make a PDC-based earth boring bit. As can be observed, the bit body 10 essentially consists of a central portion 11 having holes 12 through which mud may be pumped, as well as arms or blades 13 having pockets 14 into which the PDC cutters are attached. The bit body 10 of FIG. 1 was prepared by powder metal technologies. Typically, to prepare such a bit body, a mold is filled with powdered metals comprising both the binder metal and the carbide. The mold is then compacted to densify the powdered metal and form a green compact. Due to the strength and hardness of sintered cemented carbides, the bit body is usually machined in the green compact form. The green compact may be machined to include any features desired in the final bit body.
The overall durability and performance of fixed-cutter bits depends not only on the durability and performance of the cutting elements, but also on the durability and performance of the bit bodies. It can thus be expected that earth-boring bits based on cemented carbide bit bodies would exhibit significantly enhanced durability and performance compared with bits made using steel or infiltrated bit bodies. However, earth boring bits including solid cemented carbide bit bodies do suffer from limitations, such as the following:
1. It is often difficult to control the positions of the individual PDC cutters accurately and precisely. After machining the insert pockets, the green compact is sintered to further densify the bit body. Cemented carbide bodies will suffer from some slumping and distortion during high temperature sintering processes and this results in distortion of the location of the insert pockets. Insert pockets that are not located precisely in the designed positions of the bit body may not perform satisfactorily due to premature breakage of cutters and/or blades, drilling out-of-round holes, excessive vibration, inefficient drilling, as well as other problems.
2. Since the shapes of solid, one-piece, cemented carbide bit bodies are very complex (see for example, FIG. 1), cemented carbide bit bodies are machined and shaped from green powder compacts utilizing sophisticated machine tools. For example, five-axis computer controlled milling machines. However, even when the most sophisticated machine tools are employed, the range of shapes and designs that can be fabricated are limited due to physical limitations of the machining process. For example, the number of cutting blades and the relative positions of the PDC cutters may be limited because the different features of the bit body could interfere with the path of the cutting tool during the shaping process.
3. The cost of one-piece cemented carbide bit bodies can be relatively high since a great deal of very expensive cemented carbide material is wasted during the shaping or machining process.
4. It is very expensive to produce a one-piece cemented carbide bit body with different properties at different locations. The properties of solid, one-piece, cemented carbide bit bodies are therefore, typically, homogenous, i.e., have similar properties at every location within the bit body. From a design and durability standpoint, it may be advantageous in many instances to have different properties at different locations.
5. The entire bit body of a one-piece bit body must be discarded if a portion of the bit body fractures during service (for example, the breakage of an arm or a cutting blade).
Accordingly, there is a need for improved bit bodies for earth-boring bits having increased wear resistance, strength and toughness that do not suffer from the limitations noted above.